The absolute first step is to have a gmail account. I don't think I need to help you with that but after that things get a bit more complicated.
Syndesis is using the OAuth Standard to connect to the GoogleSheets API. On the Settings section on Syndesis it requires you to enter a Consumer Secret and Consumer Key (see Figure 1)
Figure 1. Syndesis GoogleSheets Settings
The workflow to obtain the clientID and Secret are similar to the instructions for GMail Creds but make sure to add the GoogleSheets API from the Library so that you can add it to your scope. If you are on minishift the autorized domain will be nip.io.
Figure 2. Select GoogleSheets API Scope
and also make sure to set the callback url which you can obtain from the top of the Settings screen in the Syndesis UI.
Figure 3. Set the Callback URL
You can now use the clientId and secret on this page for the Syndesis Settings for GoogleSheets shown in Figure 1. And finally you can create a GoogleSheets Connection and these credentials will be used.